TECHNOLOGY BAE SYSTEMS’ TARANIS UCAV The UK’s first true stealth aircraft, the Taranis unmanned combat air vehicle (UCAV), completed its radar cross-section (RCS) measurement programme on BAE Systems’ RCS Range at Warton, Lancashire, in mid-May, and surprisingly also made a brief, first unofficial ‘public’ appearance in the process.

This completed a prolonged period of clandestine testing under cover of night for this highly classified programme, which employed special security measures and reporting restrictions.

The Taranis programme was announced at Farnborough in 2006, and just two years ago, in a ‘first view’ prelude to Farnborough Airshow International (FAI) 2010, the UCAV technology demonstrator was revealed at Warton by the Ministry of Defence (MoD). No close inspection of the only superficially complete airframe was permitted.

In a ‘second view’ prelude to FAI 2012, Tom Fillingham, BAE Systems’ director of future combat air systems, spoke about programme progress at a media preview at Warton in mid-June. Again, although no close-up inspection was possible, Taranis, now in its definitive ‘stealth finish’, is clearly complete and “about to undergo extensive ground testing and pre-flight preparation.” Wisely, the RCS testing – an aspect of the programme arguably at least as important as the flying – has been performed prior to first flight. This is planned to take place at Woomera, Australia, the company’s favoured facility, either later this year or more likely in early 2013. Commenting on the RCS testing, Tom Fillingham said that data was now being evaluated by the MoD, Defence Science and Technology Laboratory and company specialists, “with the results looking very promising.

Taranis – in DepthIn common with other manned and unmanned low-RCS air vehicles, the blended wing/body of Taranis has remarkably smooth external contours, presumably coated with radar-absorbent material (RAM), and presently at least devoid of any protuberances.

Aerodynamic control of the low aspect ratio, highly swept, finless configuration is by means of upper and lower surface ‘drag spoilers’ on the outer wings, operating differentially port and starboard to control yaw; and large-span single-piece trailing-edge elevons acting as elevators and ailerons for pitch and roll control.

This form of control was proven on the similarly configured Raven sub-scale demonstrator which performed its first flight on December 17, 2003 (the 100th anniversary of the Wright brothers’ first flight).

The powerplant is a single Rolls-Royce Adour (non-reheat) turbofan, of some 6,000lb (26.69kN) thrust, as used in the BAE Hawk trainer family, and presumably incorporating full authority engine control (as in later marks of Adour) to facilitate integration with the vehicle flight control system. The raked triangular dorsal intake leads to a downwards-curving duct, previously tested and refined for low RCS on the Nightjar test body – this arrangement offering most scope for suppression of otherwise massive radar returns which would swamp the low signature of the basic airframe. Treatments available for such applications include RAM duct lining and radar ‘blocker’ devices, the latter in the duct or at the engine face. The basic Adour engine does not feature inlet guide vanes, a useful feature of some US engines.

A further significant feature is the absence of an intake boundary layer diverter, the latter being a problematic area for minimal RCS. The engine exhausts via a ‘letter box’ nozzle, which blends well with the overall configuration, and potentially offers lower radar and infrared signatures. However, the novel exhaust system – with strength, stiffening and cooling issues peculiar to its shape (which changes from a round form to a rectangular one) – clearly presented a challenge to Rolls-Royce and may well have accounted for some programme delay. Tom Fillingham said that a complete propulsion rig proved necessary to satisfactorily demonstrate integrity and performance, a goal that was achieved earlier this year at the Rolls-Royce Bristol facility, opening a crucial path to flight clearance. Installed engine ground running was still due to commence as of mid-June.

RCS testing has posed its own challenges. Although larger and heavier aircraft, such as Tornado and Typhoon have been tested, there is a need to obtain high-fidelity measurements of a uniquely low-signature target which has been placed on a new design of leg in order to give lower returns and less interference. The aircraft was supported at its main and nose undercarriage points by three newly-designed low-RCS legs, which incorporate optimal shaping and the usual extensive RAM cladding and ground matting. Target test aspect was varied as usual by rotating the turntable and extending the legs.

Beyond TaranisThe Taranis project emerged from the UK’s Defence Industrial Strategy (DIS) programme announced in December 2005, which was intended to ensure that UK industry acquired the technology to either go it alone on an operational UCAV – or collaborate from a position of strength. Subsequently, there’s been industrial collaboration between BAE Systems and Dassault Aviation; and agreement between the British and French governments on joint development.

In the near-term, a risk-reduction programme is under way on the Telemos medium-altitude, long-endurance (MALE) UAV (broadly akin to Mantis, and involving Mantis fl ying again) while in the longer-term a fast jet strike UCAV may emerge, with Taranis playing a major development role.

Taranis is a one-off technology demonstrator, about the size of a BAE Hawk, which is some 39ft (11.9m) in length and 31ft (9.5m) span. The confi guration lends itself to two weapons bays from which stores could presumably be released at some future stage in the programme. As previously stated by BAE Systems MD Nigel Whitehead, Taranis is “a chosen confi guration to demonstrate the technology, not the specifific range and payload”.

So how might a future UCAV differ from Taranis? Payload and radius of action will determine the size and mass. To quote from a 1960s marketing brochure for a UK strike aircraft, “experience has shown a general requirement for about 1,000nm [1,850km] radius of action” and “to carry 4,000lb [1,814kg] of weapons over this distance”. Technology has changed and, with the advent of the laser-guided bomb (LGB), the acceptable load could perhaps now be halved to, say, two 1,000lb (454kg) Enhanced Paveway IIs or four 500lb (227kg) Paveway IVs, or similar.

There were good reasons for selecting the highly-swept wing of Taranis (some 60?): RCS is thus minimised over a very wide frontal sector (some 300?) by avoiding radar return ‘spikes’ from the leading edge, but this resulted in a low aspect ratio of about 1.7. However, lift/drag ratio – the key to effi cient medium/high altitude cruise – was compromised. BAE Systems has previously released artists’ impressions of more advanced concept configurations, akin to the Northrop-Grumman X-47B/UCAS-D, the current leader in this field. The X-47B is intended for operation from US Navy carriers, and has a size and mass some two to three times that of Taranis and a 1,200nm [2,220km] radius of action. Significant features include a higher aspect ratio wing with reduced sweep for improved cruise effi ciency, as used on the Northrop B-2 Spirit, by employing advanced low-observability technology to reduce leading-edge returns, and also a more integrated intake. Engine manufacturers will also play their part and it’s been reported that Rolls-Royce has studied powerplants more suited to future larger operational UCAVs than Taranis. The practicality of in-fl ight refuelling of UAVs is also actively being studied.

Any definitive operational, stealthy and largely autonomous UCAV is not envisaged as being in production for another 15 to 20 years, a timescale typical for such advanced aircraft programmes. The UK government’s about-turn on new carriers – which means UK services will not get the F-35C carrier variant offering higher payload and greater combat radius; nor catapults and arrestor gear suitable for a non-STOVL UCAV – seems problematic, especially if the French are expecting carrier-based UCAV operations.

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